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Labradorite is an intermediate to calcic member of the plagioclase feldspar solid solution series, characterized by its distinctive iridescent optical effect known as labradorescence. This phenomenon, a schiller effect, is caused by light diffraction within submicroscopic lamellar intergrowths of two exsolved feldspar phases (albite and anorthite) within the mineral structure. The colors displayed can include blue, green, yellow, orange, red, and purple, often appearing as a flash across the mineral's surface when viewed from different angles. The base color of the mineral is typically gray, dark gray, black, or greenish-gray.
How to Identify
- Color
- Typically gray, dark gray, black, or greenish-gray, with iridescent flashes of blue, green, yellow, orange, red, and purple (labradorescence).
- Luster
- Vitreous to pearly on cleavage surfaces.
- Texture
- Typically massive, granular, or in tabular crystals within host rocks. The internal texture responsible for labradorescence is submicroscopic lamellar twinning.
- Crystal Form
- Usually anhedral to subhedral grains in igneous rocks; can form tabular to blocky crystals, often twinned (polysynthetic twinning visible on cleavage surfaces as fine parallel lines).
- Cleavage
- Two distinct cleavages at nearly 90 degrees (86° and 94°), typically perfect to good.
- Geological Environment
- Mafic igneous rocks (gabbro, basalt, anorthosite), some metamorphic rocks, and detrital sediments derived from these sources.
Key Facts
- Hardness: 6 to 6.5 on the Mohs scale
- Specific Gravity: 2.68 to 2.72
- Crystal System: Triclinic
- Color: Gray, dark gray, black, greenish-gray, with iridescent blue, green, yellow, orange, red, purple flashes
- Luster: Vitreous to pearly
- Transparency: Transparent to translucent to opaque
- Fracture: Uneven to conchoidal
- Cleavage: Perfect in two directions at nearly 90 degrees (86° and 94°)
- Composition: (Ca,Na)(Al,Si)AlSi2O8, specifically (Na,Ca)Al(Si,Al)Si2O8, with 50-70% anorthite (CaAl2Si2O8) and 30-50% albite (NaAlSi3O8) end-members.
Quick Check
- Color: Gray to dark gray with iridescent flashes (labradorescence)
- Luster: Vitreous to pearly
- Streak: White
Physical Characteristics
- Crystal Habit: Typically massive, granular, or in tabular to blocky crystals; often twinned (polysynthetic twinning).
- Cleavage Type: Perfect in two directions, forming angles of approximately 86° and 94°.
- Fracture Type: Uneven to conchoidal.
- Tenacity: Brittle.
- Luster Type: Vitreous to pearly on cleavage surfaces.
Formation
Labradorite forms primarily in mafic igneous rocks such as basalt, gabbro, and anorthosite, where it crystallizes from magma. It can also be found in some metamorphic rocks, particularly those derived from mafic protoliths, and occasionally in detrital sediments.
Usage
Primarily used as a gemstone and ornamental stone due to its striking labradorescence. It is also used in architectural applications (countertops, tiles) and as a component in some ceramics and glass manufacturing, though less commonly than other feldspars.
Age Distribution
Found in igneous and metamorphic rocks of various ages, from Precambrian to Cenozoic, depending on the specific geological setting.
Where to Find
Labrador, Canada
The original and namesake locality, known for significant deposits, particularly in anorthosite intrusions.
Finland
Produces the highest quality, intensely iridescent variety known as Spectrolite, often from anorthosite bodies.
Madagascar
Important source of gem-quality labradorite.
Norway
Found in various igneous and metamorphic complexes.
Russia
Occurrences in mafic intrusions.
United States (e.g., Oregon, New York)
Oregon is known for sunstone, a variety of labradorite/oligoclase, while New York has anorthosite bodies containing labradorite.
Australia
Found in some igneous rock formations.
Finding Tips
Look in Mafic Igneous Rocks
Target areas with known occurrences of gabbro, basalt, and especially anorthosite, as these are the primary host rocks for labradorite.
Examine Cleavage Surfaces
The characteristic labradorescence is best observed on polished or freshly broken cleavage surfaces. Rotate the sample under a light source to catch the iridescent flash.
Check for Polysynthetic Twinning
Fine, parallel striations (twinning lamellae) on cleavage surfaces are indicative of plagioclase feldspar and can help distinguish it from potassium feldspar.
Consider Associated Minerals
Labradorite often occurs with pyroxenes (augite), olivine, and magnetite in mafic rocks.
Safety Precautions
When collecting in the field, always wear appropriate safety gear, including eye protection and gloves. Be aware of your surroundings and potential hazards in geological environments. Labradorite itself is not hazardous, but the rocks it's found in can be heavy and sharp.
Similar Rocks
Andesine
Andesine
Also known as: Red Andesine
Oligoclase
Oligoclase
Also known as: Sunstone (some varieties)
Bytownite
Bytownite
Also known as: None common
Anorthite
Anorthite
Also known as: None common
Scientific Classification
- Mineral Class
- Silicate
- Group
- Feldspar Group, Plagioclase Series
- Crystal System
- Triclinic
- Chemical Formula
- (Ca,Na)(Al,Si)AlSi2O8
- Composition
- A solid solution of albite (NaAlSi3O8) and anorthite (CaAl2Si2O8), with labradorite specifically defined as having 50-70% anorthite component.
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